As the world's need for communication capacity continues to increase, the use of optical signals to transfer large amounts of information has become increasingly favored over other schemes such as those using twisted copper wires, coaxial cables, or microwave links. Optical communication systems use optical signals to carry information at high speeds over an optical path such as an optical fiber. Optical fiber communication systems are generally immune to electromagnetic interference effects, unlike the other schemes listed above. Furthermore, the silica glass fibers used in fiber optic communication systems are lightweight, comparatively low cost, and are able to carry tens, hundreds, and even thousands of gigabits per second across substantial distances.
FIG. 1 illustrates an example of a non-return-to-zero (NRZ) formatted optical signal, a return-to-zero (RZ) formatted signal, and a modulated soliton pulse train. As described in Hecht, Understanding Fiber Optics, 3rd ed., Prentice-Hall (1999), which is incorporated by reference herein, at pp. 375–380, NRZ coding is probably the most common format in fiber optic communications systems. As described in Hecht, supra, solitons are optical pulses that naturally retain their shape as they travel along an optical fiber. This is basically due to a delicate balancing act between two competing effects that degrade the transmission of other pulses, in particular, (i) chromatic dispersion, which stretches out pulses carrying a range of wavelengths as they travel along a fiber, and (ii) self-phase modulation, which spreads out the range of wavelengths as pulses pass through an optical fiber.
Soliton pulses have proved surprisingly robust in optical fibers. In a long-haul wavelength division multiplexed (WDM) optical communications system, this robustness allows for increased signal power and reduced spacing among optical amplifiers and/or regenerative repeaters. The input pulses themselves do not necessarily have to match the ideal soliton shape exactly, because fiber transmission gives them the proper soliton shape. Thus, the transmission of RZ pulses, also shown in FIG. 1, can often results in soliton propagation along the optical fiber. Even if not resulting in an ideal match to soliton propagation, the RZ pulses nevertheless generally experience improved robustness as compared to NRZ formatted optical signals.
Optical RZ transmitters, also termed optical RZ signal generators, have been developed for the purpose of receiving an electrical information signal at R bits/sec (period=T=1/R sec) and generating a corresponding optical signal modulated with an RZ-formatted envelope. The input electrical signal is most commonly provided in NRZ format. For a typical RZ transmitter, the output optical signal has a carrier frequency fc and free-space carrier wavelength λc=c/fc in an infrared region appropriate for optical communications, e.g., fc≈196.08 THz/λc=1530 nm. Modulation rates R for commercially available RZ transmitters are generally limited to R=10 Gbps (T=100 ps) or slower, although some systems with modulation rates up to 40 Gbps (T=25 ps) have been proposed.
FIG. 2 illustrates an RZ signal generator 202 in accordance with a prior art configuration that uses two amplitude modulators (AMs). RZ signal generator 202 comprises a first AM 204, a second AM 206, and a continuous wave (CW) laser 203 coupled as shown in FIG. 2. First AM 204 receives an optical carrier signal at frequency fc from CW laser 203. First AM 204 comprises a Mach-Zehnder interferometer (MZI) having a first path 208 and a second path 209, the first path 208 having no phase modulator and the second path 209 having phase modulator 210 that introduces a phase shift θ. At an output 211, the first AM 204 is designed to provide the difference between the signals present on the first path 208 and the second path 209. The phase shift θ is modulated by a sinusoidal electrical signal V1 provided by a sinusoidal signal generator 218 having a frequency equal to the desired modulation rate R=1/T, according to the relationship θ=πV1/Vπ, where Vπ is a fixed value. As known in the art, the fixed value Vπ is determined by the nature and amount of variable-refractive-index material used in the phase modulator 210. Generally speaking, when V1 equals 0 there is a “zero” phase shift (compared to an arbitrary reference value), and when V1 equals Vπ, there is a π phase shift (compared to that reference value).
Included in FIG. 2 is a plot 205 of the output optical power P1 versus input electrical voltage V1 for the first AM 204 when the input signal is an optical carrier signal at fc. When the input signal V1 is at zero, the optical signals on the first path 208 and second path 209 are identical and therefore the output power P1 is zero. When the input signal V1 approaches Vπ, these signals have a π phase difference and therefore the output power P1 is a maximum. Shown in FIG. 2 are time plots of the signal V1 and output power P1, indicating a string of narrowed optical pulses of period T being provided to the second AM 206. The second AM 206 is similar to the first AM 204, comprising a first signal path 212 and a phase modulator 214 along a second path introducing a phase shift θ that is a similar function of voltage applied as the phase modulator 210 of first AM 204. The second AM 206 receives an electrical NRZ data signal (e.g., 1101) having a magnitude normalized to Vπ. The second AM 206 simply serves as a gate for the optical pulses provided by the first AM 204, allowing a pulse to pass through when the NRZ data is a “1” and causing a zero output when the NRZ data is a “0”. FIG. 2 also includes plots of the NRZ data signals and the resulting output signal POUT.
The RZ signal generator 202 of FIG. 2 has one or more shortcomings that can reduce its effectiveness, especially at higher modulation rates above 10 Gbps. In particular, the pulse width of the output signal, measured as the time difference between successive points of 50% power (−3 dB) relative to the maximum of the pulse, is generally between 0.45 T and 0.5 T. The pulse width can be narrowed somewhat by adjusting the specific bias point of the first AM 204, i.e., the DC value of V1 in FIG. 2, or by judiciously adjusting the amplitude of the sinusoidal component of V1. Disadvantageously, however, output power levels are reduced as a result of such manipulations. Furthermore, the pulse width generally cannot be made narrower than approximately 0.42 T regardless of the output power levels. Another disadvantage is that the extinction ratio of the RZ signal generator 202, defined as the ratio between the maximum output signal power (during a “1”) and the minimum output signal power (during a “0”), will suffer substantially if the amplitude of either AM driving voltage deviates from Vπ. This is because, during intended “off” or zero-power intervals, the phase differences in the arms of the AMs will deviate from π when the voltage amplitude deviates from a zero-transmission bias point, causing unwanted non-zero output power levels during these intervals. Another disadvantage is that two amplitude modulators (AMs) are required in the RZ signal generator 202 of FIG. 2. This brings about increased system cost and complexity, each AM requiring a finely biased and electronically controlled delay element as well as a precise signal coupler.
The RZ signal generator 202 produces an output signal in which the instantaneous optical frequency finst deviates from the nominal optical frequency fc. It can be shown that the frequency shift generated by the RZ signal generator 202 can be expressed as finst−fc=(−π/4)(R)sin(2πRt). Thus, for a 10 Gbps modulation rate, the amount of frequency shift varies sinusoidally between peaks of +/−7.85 GHz.
FIG. 3 shows an RZ signal generator 302 in accordance with a prior art configuration similar to one discussed in U.S. Pat. No. 5,625,722. RZ signal generator 302 comprises an amplitude modulator (AM) 304 having a first path 307 and a second path 310, a phase shifting element 308 being placed along the first path 307 and a phase shifting element 312 being placed along the second path 310. The AM 304 receives an optical carrier signal at frequency fc from a continuous wave (CW) laser 306. The phase shifting elements 308 and 312 are symmetric with respect to each other around a bias phase shift, such that the phase shift element 308 advances the phase of the optical signal by θ1=πV1/Vπ with respect to the bias phase shift when provided with a voltage V1, and such that the phase shift element 312 retards the phase of the optical signal by that same amount when provided with the opposite voltage. Included in FIG. 3 is the resulting plot 305 of the output optical power POUT versus input electrical voltage V1 for the AM 304 when the input optical signal is a carrier at fc.
RZ signal generator 302 further comprises a differential encoder 314 for receiving the NRZ data signal and generating the input voltage V1 therefrom, and further comprises an inverter 316 for supplying (−V1) to the AM 304. The input voltage V1 is normalized to the Vπ of the AM 304. As known in the art, differential encoder 314 is a binary state machine that (i) keeps its output the same when the input is a “0”, and (ii) flips its output (0→1 or 1→0) when the input is a “1.” Included in FIG. 3 are plots of an exemplary NRZ data signal (011011), the corresponding voltage V1, and the corresponding NRZ envelope POUT of the output optical signal. As indicated in FIG. 3, the RZ signal generator 302 operates by causing a level shift in V1 whenever the input data is a “1.” As indicated by the plot 305 of the operating characteristic of AM 304, the output power is zero when the voltage V1 is at 0 or Vπ, but passes through a maximum when the voltage V1 transitions between these endpoints. Thus, when the input data is a “1” the voltage V1 will transition between endpoints, causing an optical pulse to be emitted. However, when the input data is a “0” there will be no transition in V1 and no optical pulse. It should be noted that the signal V1 will have either the curve labeled “A” or “B” in FIG. 3 depending on an initial state of the differential encoder 314, but that initial state will be irrelevant to the presence or absence of an optical pulse at the output of AM 304, which will only depend on the current value of the NRZ data stream. The RZ signal generator 302 induces no frequency shift in the optical output signal because the delay elements of the AM are symmetric with respect to each other.
The RZ signal generator 302 of FIG. 3 has one or more shortcomings that can reduce its effectiveness, especially at higher modulation rates equal to and above 10 Gbps. Although the optical pulse width can be substantially narrower than those of FIG. 2, this pulse width is a direct function of the rise time and fall time of the electrical signal V1 being provided to the AM 304. In many practical implementations, the rise and fall times of the electrical signals driving the AM 304 can be substantially different from each other, and can vary with time, temperature, or other operating conditions. This causes instability in the output pulse energies, which depend directly on these rise and fall times. For example, if the electrical rise time is a first percentage greater than the electrical fall time, then the pulse energy of adjacent optical pulses will also differ by that first percentage, which is an undesirable result. Also, if the rise and fall times vary by a second percentage due to changes in temperature or other operating condition, then the output pulse energy will also change by that second percentage, which is an undesirable result. These instabilities become increasingly problematic at high modulation rates above 10 Gbps, where these rise and fall time variations can become increasingly prominent. Another disadvantage is that the extinction ratio of the RZ signal generator 302 will also suffer substantially if the amplitude of the driving voltage V1 deviates from Vπ. This is because, during intended “off” or zero-power intervals, the phase difference in the arms of the AM 304 will deviate from π when the amplitude of V1 deviates from Vπ, causing unwanted non-zero output power levels during these intervals. Stated another way, with reference to plot 305 of FIG. 3, the voltage V1 must be maintained very close to 0 or very close to Vπ or there will be non-zero output power POUT during intended “off” intervals.
Accordingly, it would be desirable to provide an optical RZ signal generator capable of generating a reliable stream of RZ optical pulses corresponding to an electrical information signal.
It would be further desirable to provide an optical RZ signal generator that can provide narrow optical pulses having increased pulse width stability.
It would be still further desirable to provide an optical RZ signal generator that is cost-effective in terms of the number of high-cost precision components required.
It would be even further desirable to provide an optical RZ signal generator in which the pulse width can be adjustable, either at the factory or dynamically during operation.
It would be even further desirable to provide an optical RZ signal generator in which the extinction ratio of the output optical signal has increased stability with respect to variations in the amplitude of the electrical signals driving its electro-optical components.
It would be still further desirable to provide an optical RZ signal generator that is readily amenable to single-chip integration, dual-chip integration, and/or integration with downstream optical components such as optical amplifiers or optical attenuators.
It would be even further desirable to provide a system and method for integrating a plurality of optical components including interferometers onto smaller substrate areas.